Description

The operating current flow in a field-effect transistor is controlled by the same sign charge carriers (electrons or holes), so such devices are called unipolar (as opposed to bipolar).

According to the physical structure and the operational principle, field-effect transistors are divided into two groups. The first are p-n junction transistors or metal-semiconductor junction (Schottky barrier), the second are insulated electrode (gate) transistors, so-called MIS (metal-insulator-semiconductor) or MOS (metal-oxide-semiconductor) transistors. See Figure for the principle of field-effect transistor operation.

FETs can be divided into four main groups by area of application: FET for digital devices and integrated circuits, FET of general application, microwave FET and high power FET.

FETs designed for digital devices and integrated circuits should be small-size, with high switching rate and the minimum switching energy. Such transistors may be based on silicon or GaAs. The best performance was observed in the case of heterostructure-based FETs with selective doping, MODFETs. MODFETs, also known as high electron mobility transistors (HEMT, HFET), use the properties of the two-dimensional electron gas that forms in some heterostructures on the boundary of the narrow-bandgap and wide-bandgap layers of a heteropair.

The main requirement for a microwave-FET is to achieve maximum power or gain at an extremely high frequency. To advance further in the field of high frequencies requires reduced gate length to be ensured, as well as maximising utilisation of ballistic effects in order to achieve high-speed media.

High-power FETs have a great overall electrode length because the capacity per unit of the structure’s operation area is fundamentally limited by the need to remove heat.

Illustrations

а — Thin semiconductor plate (channel) contains two ohmic electrodes (source and drain). Another electrode (gate) is located between the source and the drain. b — When voltage is applied between the gate and either of the other two electrodes (source of drain), an electric field is generated under the gate of the channel. This field changes the quantity of charge carriers in the channel near the gate, hence changing the channel's resistance.